This invention relates generally in the field of holography and is specifically directed, although not restricted, to a method of producing wide angle white light viewable holograms.
A simple form of classical holography requires that one record the coherent light reflected from a three-dimensional object. If there is insufficient light available from the laser source which illuminates the object, or if the object moves by even the slightest amount during the recording, then this method of holography is not useful. A technique is described in Scientific American, Oct. 1976, pages 80 to 95, in which a laser light source is not required. Furthermore with this method, known as composite holography (sometimes referred to as multiplex or lenticular holography and herein referred to as conventional composite holography) the restriction on object motion is not severe. With composite holography, the hologram is synthesized from a multiplicity of conventional incoherent light photographs each captured by a camera positioned at a slightly different azimuthal viewing angle. Typically 1080 such photographs are used, each recorded at angles displaced from each other by one-third of a degree. Each photograph is illuminated with coherent laser light and is used as the object in a tall but narrow elemental hologram. These elemental holograms are placed side by side in an order which corresponds to the viewing angle of their respective photographs. The hologram can be reconstructed using white light. The image lacks vertical parallax but retains the visual effects of the three dimensions if a large enough number of photographs are used, since horizontal parallax is retained. If the object is photographed throughout angles surrounding the object, the resulting hologram is a drum which surrounds the virtual image. Images reconstructed from these holograms contain unavoidable distortions. These distortions come about for the following reason. One can regard the construction of composite holograms as an attempt to capture and reconstruct views of the object (albeit demagnified in most cases) which are seen by the camera in the photography step. This being the case, an observer must observe the image through a rather short cylinder, the diameter of which represents the horizontal trajectory of the camera. The image viewed from a composite hologram is undistorted only when this hologram is very short in comparison with the image height, and from which the image rays are greatly diverging. Such a procedure results unfortunately in a hologram whose imagery is unacceptable for most viewing conditions. Conventional composite holograms are in fact quite tall (equal approximately to the image height) a necessity since they must simulate a vertical window of adequate size. Failure to create holograms so short that they eliminate distortions is not particularly disastrous for conventional composite holograms which are viewed from large distances. But it accounts for an inherent image distortion which is quite evident when viewing from locations close to the hologram surface.
The optical system used to convert the photographic transparencies to the hologram, as well as those systems used for the original photography, use lens elements with practical limitations (F numbers &gt;1) and because of this, the image is prohibited from occuping a space within the cylinder which is greater than half the cylinder diameter. Furthermore, composite hologram configurations other than cylindrical, such as flat plates, cannot be generated with this system.
Cylindrical holograms of stationary objects can be formed using classical methods by illuminating a stationary object with coherent light and surrounding the object with undeveloped photographic film. The film is exposed with the object reflected light and with a suitable reference. The images in such holograms again cannot occupy a large volume of the resulting drum because of the physical constraints in positioning this construction reference beam.
A method of copying original holograms, which has been frequently used in the past, is similar to that discussed by Rotz and Freisem in Applied Physics Letters, Vol. 8, No. 6, page 146 (March 1966). The implementation of the method for copying the cylindrical holograms requires that one cut the cylinder so that it may be located as a flat plate, and then reilluminate it with a suitably converging, diverging, or plane reference wave. The diffracted image wave and a suitable reference wave are intercepted at some distance, perhaps a very small distance from this original hologram, by a second holographic surface on which is constructed a second hologram. While this procedure seems like an obvious method of creating copy cylindrical holograms, it is in fact limited in that the viewing angle is reduced rather than increased in the copy image. Cylindrical holograms copied by the method create images which occupy no greater a percentage of the cylindrical volume than that of the original.
A second and related method of copying holograms is a contact copy method described by Brumm in Applied Optics, Vol. 5, No. 12, page 1946 (Dec. 1966). This method is commonly used for copying composite cylindrical holograms. The copy is created with a second film placed very close to the original such that the construction reference wave for the second hologram is the undiffracted portion of the original hologram reillumination wave. However, this wave is unavoidably contaminated on passing through this first hologram and contributes to noise in the image of the second hologram. The object wave for the second hologram is the image diffracted from the first. Thus, one has no method of optimizing the reference to object beam ratio in the second hologram construction. The image in the copy retains the limited viewing angles of the original.
Both of these copy methods allow one, in principle, to create copies with a single exposure, rather than with the multiple (for example 1080) exposures required for the composite original. This single exposure requirement is a necessity if efficient production is to be maintained.
It is an object of the present invention to construct cylindrical holograms whose images occupy the entire volume within the cylinder.
It is another object of this invention to construct second holograms from original cylindrical holograms whose configurations are other than cylindrical, such as flat plates, and which retain the large horizontal viewing angles of the original cylindrical holograms.
It is a further object of this invention to construct second holograms from cylindrical originals in which the holographic recording surface or volume lies within the image space.
It is yet another object of this invention to construct in a single laser exposure, copy holograms from original cylindrical holograms which contain distortion free images.